Video segmentation using statistical pixel modeling

ABSTRACT

A method for segmenting video data into foreground and background portions utilizes statistical modeling of the pixels. A statistical model of the background is built for each pixel, and each pixel in an incoming video frame is compared with the background statistical model for that pixel. Pixels are determined to be foreground or background based on the comparisons. The method for segmenting video data may be further incorporated into a method for implementing an intelligent video surveillance system.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. application Ser. No.09/815,385 now Pat. No. 6,625,310, filed on Mar. 23, 2001,commonly-assigned, and incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to processing of video frames for use invideo processing systems, for example, intelligent video surveillance(IVS) systems that are used as a part of or in conjunction with ClosedCircuit Television Systems (CCTV) that are utilized in security,surveillance and related homeland security and anti-terrorism systems,IVS systems that process surveillance video in retail establishments forthe purposes of establishing in-store human behavior trends for marketresearch purposes, IVS systems that monitor vehicular traffic to detectwrong-way traffic, broken-down vehicles, accidents and road blockages,and video compression systems. IVS systems are systems that furtherprocess video after video segmentation steps to perform objectclassification in which foreground objects may be classified as ageneral class such as animal, vehicle, or other moving but-unclassifiedobject, or may be classified in more specific classes as human, small-or large-non-human animal, automobile, aircraft, boat, truck, tree,flag, or water region. In IVS systems, once such video segmentation andclassification occurs, then detected objects are processed to determinehow their positions, movements and behaviors relate to user definedvirtual video tripwires, and virtual regions of interest (where a regionof interest may be an entire field of view, or scene). User definedevents that occur will then be flagged as events of interest that willbe communicated to the security officer or professional on duty.Examples of such events include a human or a vehicle crossing a virtualvideo tripwire, a person or vehicle loitering or entering a virtualregion of interest or scene, or an object being left behind or takenaway from a virtual region or scene. In particular, the presentinvention deals with ways of segmenting video frames into theircomponent parts using statistical properties of regions comprising thevideo frames.

BACKGROUND OF THE INVENTION

In object-based video compression, video segmentation for detecting andtracking video objects, as well as in other types of object-orientedvideo processing, the input video is separated into two streams. Onestream contains the information representing stationary backgroundinformation, and the other stream contains information representing themoving portions of the video, to be denoted as foreground information.The background information is represented as a background model,including a scene model, i.e., a composite image composed from a seriesof related images, as, for example, one would find in a sequence ofvideo frames; the background model may also contain additional modelsand modeling information. Scene models are generated by aligning images(for example, by matching points and/or regions) and determining overlapamong them; generation of scene models is discussed in further depth incommonly-assigned U.S. patent application Ser. No. 09/472,162, filedDec. 27, 1999, and Ser. No. 09/609,919, filed Jul. 3, 2000, bothincorporated by reference in their entireties herein. In an efficienttransmission or storage scheme, the scene model need be transmitted onlyonce, while the foreground information is transmitted for each frame.For example, in the case of an observer (i.e., camera or the like, whichis the source of the video) that undergoes only pan, tilt, roll, andzoom types of motion, the scene model need be transmitted only oncebecause the appearance of the scene model does not change from frame toframe, except in a well-defined way based on the observer motion, whichcan be easily accounted for by transmitting motion parameters. Note thatsuch techniques are also applicable in the case of other forms ofmotion, besides pan, tilt, roll, and zoom. In IVS systems, the creationof distinct moving foreground and background objects allows the systemto attempt classification on the moving objects of interest, even whenthe background pixels may be undergoing apparent motion due to pan, tiltand zoom motion of the camera.

To make automatic object-oriented video processing feasible, it isnecessary to be able to distinguish the regions in the video sequencethat are moving or changing and to separate (i.e., segment) them fromthe stationary background regions. This segmentation must be performedin the presence of apparent motion, for example, as would be induced bya panning, tilting, rolling, and/or zooming observer (or due to othermotion-related phenomena, including actual observer motion). To accountfor this motion, images are first aligned; that is, correspondinglocations in the images (i.e., frames) are determined, as discussedabove. After this alignment, objects that are truly moving or changing,relative to the stationary background, can be segmented from thestationary objects in the scene. The stationary regions are then used tocreate (or to update) the scene model, and the moving foreground objectsare identified for each frame.

It is not an easy thing to identify and automatically distinguishbetween video objects that are moving foreground and stationarybackground, particularly in the presence of observer motion, asdiscussed above. Furthermore, to provide the maximum degree ofcompression or the maximum fineness or accuracy of other videoprocessing techniques, it is desirable to segment foreground objects asfinely as possible; this enables, for example, the maintenance ofsmoothness between successive video frames and crispness withinindividual frames. Known techniques have proven, however, to bedifficult to utilize and inaccurate for small foreground objects andhave required excessive processing power and memory. It would,therefore, be desirable to have a technique that permits accuratesegmentation between the foreground and background information andaccurate, crisp representations of the foreground objects, without thelimitations of prior techniques.

SUMMARY OF THE INVENTION

The present invention is directed to a method for segmentation of videointo foreground information and background information, based onstatistical properties of the source video. More particularly, themethod is based on creating and updating statistical informationpertaining to a characteristic of regions of the video and the labelingof those regions (i.e., as foreground or background) based on thestatistical information. For example, in one embodiment, the regions arepixels, and the characteristic is chromatic intensity. Many otherpossibilities exist, as will become apparent. In more particularembodiments, the invention is directed to methods of using the inventivevideo segmentation methods to implement intelligent video surveillancesystems.

In embodiments of the invention, a background model is developedcontaining at least two components. A first component is the scenemodel, which may be built and updated, for example, as discussed in theaforementioned U.S. patent applications. A second component is abackground statistical model.

In a first embodiment, the inventive method comprises a two-pass processof video segmentation. The two passes of the embodiment comprise a firstpass in which a background statistical model is built and updated and asecond pass in which regions in the frames are segmented. An embodimentof the first pass comprises steps of aligning each video frame with ascene model and updating the background statistical model based on thealigned frame data. An embodiment of the second pass comprises, for eachframe, steps of labeling regions of the frame and performing spatialfiltering.

In a second embodiment, the inventive method comprises a one-passprocess of video segmentation. The single pass comprises, for each framein a frame sequence of a video stream, steps of aligning the frame witha scene model; building a background statistical model; labeling theregions of the frame, and performing spatial/temporal filtering.

In yet another embodiment, the inventive method comprises a modifiedversion of the aforementioned one-pass process of video segmentation.This embodiment is similar to the previous embodiment, except that thestep of building a background statistical model is replaced with a stepof building a background statistical model and a secondary statisticalmodel.

Each of these embodiments may be embodied in the forms of a computersystem running software executing their steps and a computer-readablemedium containing software representing their steps.

DEFINITIONS

In describing the invention, the following definitions are applicablethroughout (including above).

A “computer” refers to any apparatus that is capable of accepting astructured input, processing the structured input according toprescribed rules, and producing results of the processing as output.Examples of a computer include: a computer; a general purpose computer;a supercomputer; a mainframe; a super mini-computer; a mini-computer; aworkstation; a micro-computer; a server; an interactive television; ahybrid combination of a computer and an interactive television; andapplication-specific hardware to emulate a computer and/or software. Acomputer can have a single processor or multiple processors, which canoperate in parallel and/or not in parallel. A computer also refers totwo or more computers connected together via a network for transmittingor receiving information between the computers. An example of such acomputer includes a distributed computer system for processinginformation via computers linked by a network.

A “computer-readable medium” refers to any storage device used forstoring data accessible by a computer. Examples of a computer-readablemedium include: a magnetic hard disk; a floppy disk; an optical disk,like a CD-ROM or a DVD; a magnetic tape; a memory chip; and a carrierwave used to carry computer-readable electronic data, such as those usedin transmitting and receiving e-mail or in accessing a network.

“Software” refers to prescribed rules to operate a computer. Examples ofsoftware include: software; code segments; instructions; computerprograms; and programmed logic.

A “computer system” refers to a system having a computer, where thecomputer comprises a computer-readable medium embodying software tooperate the computer.

A “network” refers to a number of computers and associated devices thatare connected by communication facilities. A network involves permanentconnections such as cables or temporary connections such as those madethrough telephone or other communication links. Examples of a networkinclude: an internet, such as the Internet; an intranet; a local areanetwork (LAN); a wide area network (WAN); and a combination of networks,such as an internet and an intranet.

“Video” refers to motion pictures represented in analog and/or digitalform. Examples of video include video feeds from CCTV systems insecurity, surveillance and anti-terrorism applications, television,movies, image sequences from a camera or other observer, andcomputer-generated image sequences. These can be obtained from, forexample, a wired or wireless live feed, a storage device, a firewireinterface, a video digitizer, a video streaming server, device orsoftware component, a computer graphics engine, or a network connection.

“Video processing” refers to any manipulation of video, including, forexample, compression and editing.

A “frame” refers to a particular image or other discrete unit within avideo.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in further detail in connection withthe attached drawings, in which:

FIG. 1 shows a flowchart corresponding to an implementation of a firstembodiment of the invention;

FIGS. 2 a and 2 b show flowcharts corresponding to two alternativeembodiments of the labeling step in the flowchart of FIG. 1;

FIGS. 3 a and 3 b show flowcharts corresponding to implementations ofthe spatial/temporal filtering step in the flowchart of FIG. 1;

FIG. 4 shows a flowchart corresponding to an implementation of a secondembodiment of the invention;

FIG. 5 shows a flowchart corresponding to an implementation of one ofthe steps in the flowchart of FIG. 4;

FIGS. 6 a and 6 b together show a flowchart corresponding to animplementation of another one of the steps in the flowchart of FIG. 4;

FIG. 7 shows a flowchart corresponding to an implementation of a thirdembodiment of the invention;

FIGS. 8 a and 8 b together show a flowchart corresponding to animplementation of one of the steps in the flowchart of FIG. 7;

FIG. 9 depicts an embodiment of the invention in the form of softwareembodied on a computer-readable medium, which may be part of a computersystem; and

FIG. 10 depicts a flowchart of a method of implementing an intelligentvideo surveillance system according to an embodiment of the invention.

Note that identical objects are labeled with the same reference numeralsin all of the drawings that contain them.

DETAILED DESCRIPTION OF THE INVENTION

As discussed above, the present invention is directed to thesegmentation of video streams into foreground information, whichcorresponds to moving objects, and background information, whichcorresponds to the stationary portions of the video. The presentinvention may be embodied in a number of ways, of which three specificones are discussed below. These embodiments are meant to be exemplary,rather than exclusive.

The ensuing discussion refers to “pixels” and “chromatic intensity;”however, the inventive method is not so limited. Rather, the processingmay involve any type of region (including regions comprising multiplepixels), not just a pixel, and may use any type of characteristicmeasured with respect to or related to such a region, not just chromaticintensity.

1. First Embodiment—Two-Pass Segmentation

The first embodiment of the invention is depicted in FIG. 1 andcorresponds to a two-pass method of segmentation. As shown in FIG. 1,the method begins by obtaining a frame (or video) sequence from a videostream (Step 1). The frame sequence preferably includes two or moreframes of the video stream. The frame sequence can be, for example, aportion of the video stream or the entire video stream. As a portion ofthe video stream, the frame sequence can be, for example, one continuoussequence of frames of the video stream or two or more discontinuoussequences of frames of the video stream. As part of the alignment step,the scene model is also built and updated.

After Step 1, in Step 2, it is determined whether or not all frames haveyet been processed. If not, the next frame is taken and aligned with theunderlying scene model of the video stream (Step 3); such alignment isdiscussed above, and more detailed discussions of alignment techniquesmay be found, for example, in commonly-assigned U.S. patent applicationSer. No. 09/472,162, filed Dec. 27, 1999, and Ser. No. 09/609,919, filedJul. 3, 2000, both incorporated by reference in their entireties herein,as discussed above, as well as in numerous other references.

The inventive method is based on the use of statistical modeling todetermine whether a particular pixel should be classified as being aforeground object or a part thereof or as being the background or a partthereof. Step 4 deals with the building and updating of a statisticalmodel of the background, using each frame aligned in Step 3.

The statistical model of the present invention comprises first- andsecond-order statistics. In the ensuing discussion, mean and standarddeviation will be used as such first- and second-order statistics;however, this is meant to be merely exemplary of the statistics that maybe used.

In general, the mean of N samples, x, is computed by taking the sum ofthe samples and dividing it by N, i.e.,

$\begin{matrix}{{\overset{\_}{x} = \frac{\sum\limits_{i = 1}^{N}X_{i}}{N}},} & (1)\end{matrix}$where x_(i) is a particular sample corresponding to a given pixel (orregion), which in the present case could be, for example, the measuredchromatic intensity of the i^(th) sample corresponding to the givenpixel (or region). In the present setting, then, such a mean would becomputed for each pixel or region.

While Eqn. (1) gives the general formula for a sample mean, it may notalways be optimal to use this formula. In video processing applications,a pixel's sample value may change drastically when an object movesthrough the pixel and change (drastically) back to a value around itsprevious value after the moving object is no longer within that pixel.In order to address this type of consideration, the invention utilizes aweighted average, in which the prior values are weighted more heavilythan the present value. In particular, the following equation may beused:x _(N) =W _(p) x _(N−1) +W _(n) x _(N),  (2)where W_(p) is the weight of the past values and W_(n) is the weightassigned to the newest value. Additionally, x _(J) represents theweighted average taken over J samples, and x_(K) represents the K^(th)sample. W_(p) and W_(n) may be set to any pair of values between zeroand one such that their sum is one and such that W_(n)<W_(p), so as toguarantee that the past values are more heavily weighted than the newestvalue. As an example, the inventors have successfully used W_(p)=0.9 andW_(n)=0.1.

Standard deviation, σ, is determined as the square root of the variance,σ², of the values under consideration. In general, variance isdetermined by the following formula:σ²= x ² −( x )²,  (3)where x² represents the average of x²; thus, the standard deviation isgiven byσ=√{square root over ( x ² −( x )².)}  (4)Because the inventive method uses running statistics, this becomesσ_(N)=√{square root over ({ x ² }_(N)−( x _(N) )²)},   (4a)where x_(N) is as defined in Eqn. (2) above, and { x² }_(N) is definedas the weighted ave of the squared values of the samples, through theN^(th) sample, and is given by{ x ² }_(N) =W _(P){ x ² }_(N−1) +W _(n) x _(N) ².  (5)As in the case of the weighted average of the sample values, the weightsare used to assure that past values are more heavily weighted than thepresent value.

Given this, Step 4 works to create and update the statistical model bycomputing the value of Eqn. (4a) for each pixel, for each frame. In Step4, the values for the pixels are also stored on a pixel-by-pixel basis(as opposed to how they are received, i.e., on a frame-by-frame basis);that is, an array of values is compiled for each pixel over the sequenceof frames. Note that in an alternative embodiment, Step 4 only performsthis storage of values.

Following Step 4, the method returns to Step 2 to check whether or notall of the frames have been processed. If they have, then the methodproceeds to Step 5, which commences the second pass of the embodiment.

In Step 5, the statistical background model is finalized. This is doneby using the stored values for each pixel and determining their mode,the mode being the value that occurs most often. This may beaccomplished, for example, by taking a histogram of the stored valuesand selecting the value for which the histogram has the highest value.The mode of each pixel is then assigned as the value of the backgroundstatistical model for that pixel.

Following Step 5, the method proceeds to Step 6, which determineswhether or not all of the frames have been processed yet. If not, thenthe method proceeds to Step 7, in which each pixel in the frame islabeled as being a foreground (FG) pixel or a background (BG) pixel. Twoalternative embodiments of the workings of this step are shown in theflowcharts of FIGS. 2 a and 2 b.

FIG. 2 a depicts a two decision level method. In FIG. 2 a, the pixellabeling Step 7 begins with Step 71, where it is determined whether ornot all of the pixels in the frame have been processed. If not, then themethod proceeds to Step 72 to examine the next pixel. Step 72 determineswhether or not the pixel matches the background statistical model, i.e.,whether the value of the pixel matches the mode for that pixel. This isperformed by taking the absolute difference between the pixel value andthe value of the background statistical model for the pixel (i.e., themode) and comparing it with a threshold; that is,Δ=|x _(pixel) −m _(pixel)|  (6)is compared with a threshold θ. In Eqn. (6), x_(pixel) denotes the valueof the pixel, while m_(pixel) represents the value of the statisticalbackground model for that pixel.

The threshold θ may be determined in many ways. For example, it may betaken to be a function of standard deviation (of the given pixel), σ. Ina particular exemplary embodiment, θ=3σ; in another embodiment, θ=Kσ,where K is chosen by the user. As another example, θ may be assigned apredetermined value (again, for each pixel) or one chosen by the user.

If Δ≦θ, then the pixel value is considered to match the backgroundstatistical model. In this case, the pixel is labeled as background (BG)in Step 73, and the algorithm proceeds back to Step 71. Otherwise, ifΔ>θ, then the pixel value is considered not to match the backgroundstatistical model, and the pixel is labeled as foreground (FG) in Step74. Again, the algorithm then proceeds back to Step 71. If Step 71determines that all of the pixels (in the frame) have been processed,then Step 7 is finished.

FIG. 2 b depicts a three decision level method, labeled 7′. In FIG. 2 b,the process once again begins with Step 71, a step of determiningwhether or not all pixels have yet been processed. If not, the processconsiders the next pixel to be processed and executes Step 72, the stepof determining whether or not the pixel being processed matches thebackground statistical model; this is done in the same way as in FIG. 2a. If yes, then the pixel is labeled as BG (Step 73), and the processloops back to Step 71. If not, then the process proceeds to Step 75;this is where the process of FIG. 2 b is distinguished from that of FIG.2 a.

In Step 75, the process determines whether or not the pixel underconsideration is far from matching the background statistical model.This is accomplished via a threshold test similar to Step 72, only inStep 75, θ is given a larger value. As in Step 72, θ may beuser-assigned or predetermined. In one embodiment, θ=Nσ, where N is aeither a predetermined or user-set number, N>K. In another embodiment,N=6.

If the result of Step 75 is that Δ≦θ, then the pixel is labeled as FG(Step 74). If not, then the pixel is labeled definite foreground (DFG),in Step 76. In each case, the process loops back to Step 71. Once Step71 determines that all pixels in the frame have been processed, Step 7′is complete.

Returning to FIG. 1, once all of the pixels of a frame have beenlabeled, the process proceeds to Step 8, in which spatial/temporalfiltering is performed. While shown as a sequential step in FIG. 1, Step8 may alternatively be performed in parallel with Step 7. Details ofStep 8 are shown in the flowcharts of FIGS. 3 a and 3 b.

In FIG. 3 a, Step 8 commences with a test as to whether or not all thepixels of the frame have been processed (Step 81). If not, in Step 85,the algorithm selects the next pixel, P_(i), for processing and proceedsto Step 82, where it is determined whether or not the pixel is labeledas BG. If it is, then the process goes back to Step 81. If not, then thepixel undergoes further processing in Steps 83 and 84.

Step 83, neighborhood filtering, is used to correct for misalignmentswhen the images are aligned. If the current image is slightly misalignedwith the growing background statistical model, then, particularly nearstrong edges, the inventive segmentation procedure, using the backgroundstatistical model, will label pixels as foreground. Neighborhoodfiltering will correct for this. An embodiment of Step 83 is depicted inthe flowchart of FIG. 3 b.

In FIG. 3 b, Step 83 begins with Step 831, where a determination is madeof the scene model location, P_(m), corresponding to P_(i). Next, aneighborhood, comprising the pixels, P′_(m), surrounding P_(m) in thescene model, is selected (Step 832). Step 833 next determines if all ofthe pixels in the neighborhood have been processed. If yes, Step 83 iscomplete, and the label of P_(i) remains as it was; if not, the processproceeds to Step 834, where the next neighborhood pixel P′_(m) isconsidered. Step 835 then tests to determine whether or not P_(i)matches P′_(m). This matching test is accomplished by executing thelabeling step (Step 7 or 7′) in a modified fashion, using P_(i) as thepixel under consideration and P′_(m) as the “corresponding” backgroundstatistical model point. If the labeling step returns a label of FG orDFG, there is no match, whereas if it returns a label of BG, there is amatch. If there is no match, the process loops back to Step 833; ifthere is a match, then this is an indication that P_(i) might bemislabeled, and the process continues to Step 836. In Step 836, aneighborhood, comprising the pixels, P′_(i), surrounding P_(i) in theframe, is selected, and an analogous process is performed. That is, inStep 833, it is determined whether or not all of the pixels, P′_(i) inthe neighborhood have yet been considered. If yes, then Step 83 iscomplete, and the label of P_(i) remains as it was; if not, then theprocess proceeds to Step 838, where the next neighborhood pixel, P′_(i),is considered. Step 839 tests to determine if P_(m) matches P′_(i); thisis performed analogously to Step 833, with the P′_(i) underconsideration being used as the pixel being considered and P_(m) as its“corresponding” background statistical model point. If it does not, thenthe process loops back to Step 837; if it does, then P_(i) is relabeledas BG, and Step 83 is complete.

Returning to FIG. 3 a, following Step 83, Step 84 is executed, in whichmorphological erosions and dilations are performed. First, apredetermined number, n, of erosions are performed to remove incorrectlylabeled foreground. Note that pixels labeled DFG may not be erodedbecause they represent either a pixel that is almost certainlyforeground. This is followed by n dilations, which restore the pixelsthat were correctly labeled as foreground but were eroded. Finally, asecond predetermined number, m, of dilations are performed to fill inholes in foreground objects. The erosions and dilations may be performedusing conventional erosion and dilation techniques, applied inaccordance with user-specified parameters, and modified, as discussedabove, such that pixels labeled DFG are not eroded.

In alternative embodiments, Step 84 may comprise filtering techniquesother than or in addition to morphological erosions and dilations. Ingeneral, Step 84 may employ any form or forms of spatial and/or temporalfiltering.

Returning to FIG. 1, following Step 8, the algorithm returns to Step 6,to determine whether or not all frames have been processed. If yes, thenthe processing of the frame sequence is complete, and the process ends(Step 9).

This two-pass embodiment has the advantage of relative simplicity, andit is an acceptable approach for applications not requiring immediate orlow-latency processing. Examples of such applications include off-linevideo compression and non-linear video editing and forensic processingof security and surveillance video. On the other hand, many otherapplications such as video security and surveillance in which timelyevent reporting is critical do have such requirements, and theembodiments to be discussed below are tailored to address theserequirements.

2. Second Embodiment—One-Pass Segmentation

FIG. 4 depicts a flowchart of a one-pass segmentation process, accordingto a second embodiment of the invention. Comparing FIG. 4 with FIG. 1(the first embodiment), the second embodiment differs in that there isonly a single pass of processing for each frame sequence. This singlepass, as shown in Steps 2, 3, 31, 32, 8 in FIG. 4, incorporates theprocesses of the second pass (Steps 5–8 in FIG. 1) with the first pass(Steps 2–4 in FIG. 1), albeit in a modified form, as will be discussedbelow.

As in the case of the first embodiment, the second embodiment (one-passprocess), shown in FIG. 4, begins by obtaining a frame sequence (Step1). As in the first embodiment, the process then performs a test todetermine whether or not all of the frames have yet been processed (Step2). Also as in the first embodiment, if the answer is no, then the nextframe to be processed is aligned with the scene model (Step 3). Asdiscussed above, the scene model component of the background model isbuilt and updated as part of Step 3, so there is always at least adeterministically-determined value in the background model at eachlocation.

At this point, the process includes a step of building a backgroundstatistical model (Step 31). This differs from Step 4 of FIG. 1, and isdepicted in further detail in FIG. 5. The process begins with a step ofdetermining whether or not all pixels in the frame being processed havebeen processed (Step 311). If not, then the process determines whetheror not the background statistical model is “mature” (Step 312) and“stable” (Step 313).

The reason for Steps 312 and 313 is that, initially, the statisticalbackground model will not be sufficiently developed to make accuratedecisions as to the nature of pixels. To overcome this, some number offrames should be processed before pixels are labeled (i.e., thebackground statistical model should be “mature”); in one embodiment ofthe present invention, this is a user-defined parameter. This may beimplemented as a “look-ahead” procedure, in which a limited number offrames are used to accumulate the background statistical model prior topixel labeling (Step 32 in FIG. 4).

While simply processing a user-defined number of frames may suffice toprovide a mature statistical model, stability is a second concern (Step313), and it depends upon the standard deviation of the backgroundstatistical model. In particular, as will be discussed below, thestatistical background model includes a standard deviation for eachpixel. The statistical model (for a particular pixel) is defined ashaving become “stable” when its variance (or, equivalently, its standarddeviation) is reasonably small. In an embodiment of the presentinvention, Step 313 determines this by comparing the standard deviationwith a user-defined threshold parameter; if the standard deviation isless than this threshold, then the statistical background model (forthat pixel) is determined to be stable.

As to the flow of Step 31, in FIG. 5, if the background statisticalmodel is determined to be mature (Step 312), it is determined whether ornot the background statistical model is stable (Step 313). If either ofthese tests (Steps 312 and 313) fails, the process proceeds to Step 315,in which the background statistical model of the pixel being processedis updated using the current value of that pixel. Step 315 will beexplained further below.

If the background statistical model is determined to be both mature andstable (in Steps 312 and 313), the process proceeds to Step 314, whereit is determined whether or not the pixel being processed matches thebackground statistical model. If yes, then the background statisticalmodel is updated using the current pixel value (Step 315); if no, thenthe process loops back to Step 311 to determine if all pixels in theframe have been processed.

Step 314 operates by determining whether or not the current pixel valueis within some range of the mean value of the pixel, according to thecurrent background statistical model. In one embodiment of theinvention, the range is a user-defined range. In yet another embodiment,it is determined to be a user-defined number of standard deviations;i.e., the pixel value, x, matches the background statistical model if|x _(pixel) − x_(pixel) |≦ Kσ,  (7)where K is the user-defined number of standard deviations, σ; x_(pixel)is the current pixel value; and x_(pixel) is the mean value of thecurrent pixel in the background statistical model. The purpose ofperforming Step 314 is to ensure, to the extent possible, that onlybackground pixels are used to develop and update the backgroundstatistical model.

In Step 315, the background statistical model is updated. In thisembodiment, the background statistical model consists of the mean andstandard deviation of the values for each pixel (over the sequence offrames). These are computed according to Eqns. (2) and (4a) above.

Following Step 315, the process loops back to Step 311, to determine ifall pixels (in the current frame) have been processed. Once all of thepixels have been processed, the process proceeds to Step 316, where thebackground statistical model is finalized. This finalization consists ofassigning to each pixel its current mean value and standard deviation(i.e., the result of processing all of the frames up to that point).

Note that it is possible for the background statistical model for agiven pixel never to stabilize. This generally indicates that theparticular pixel is not a background pixel in the sequence of frames,and there is, therefore, no need to assign it a value for the purposesof the background statistical model. Noting that, as discussed above, ascene model is also built and updated, there is always at least adeterministically-determined value associated with each pixel in thebackground model.

Following Step 316, the process goes to Step 32, as shown in FIG. 4,where the pixels in the frame are labeled according to their type (i.e.,definite foreground, foreground or background). Step 32 is shown infurther detail in the flowchart of FIGS. 6 a and 6 b.

The following concepts are embodied in the description of Step 32 tofollow. Ideally, labeling would always be done by testing each pixelagainst its corresponding point in the background statistical model, butthis is not always possible. If the background statistical model is notready to use on the basis of number of frames processed (i.e.,“mature”), then the process must fall back on testing against thecorresponding point in the scene model. If the background statisticalmodel is ready to use but has not yet settled down (i.e., is not“stable”), this is a sign that the pixel is varying and should belabeled as being foreground. If the background statistical model has,for some reason (i.e., because it fails to match the scene model orbecause it has become unsettled again), become unusable, the processmust once again fall back on testing against the scene model.

As shown in FIG. 6 a, Step 32 begins with Step 321, where it isdetermined whether or not all pixels (in the current frame) have beenprocessed. If yes, Step 32 is complete; if not, the next pixel isprocessed in Steps 322 et seq.

Step 322 determines whether or not the background statistical model ismature. This is done in the same manner as in Step 312 of FIG. 5,discussed above. If not, the process proceeds to Step 323, where it isdetermined whether or not the pixel matches the background chromaticdata of the corresponding point of the scene model.

Step 323 is performed by carrying out a test to determine whether or notthe given pixel falls within some range of the background chromatic datavalue. This is analogous to Step 314 of FIG. 5, substituting thebackground chromatic data value for the statistical mean. The thresholdmay be determined in a similar fashion (predetermined, user-determined,or the like).

If Step 323 determines that the pixel does match the backgroundchromatic data, then the pixel is labeled BG (following connector A) inStep 329 of FIG. 6 b. From Step 329, the process loops back (viaconnector D) to Step 321.

If Step 323 determines that the pixel does not match the backgroundchromatic data, then the pixel is labeled FG (following connector B) inStep 3210 of FIG. 6 b. From the Step 3210, the process loops back (viaconnector D) to Step 321.

If Step 322 determines that the background statistical model is mature,processing proceeds to Step 324, which determines whether or not thebackground statistical model is stable. Step 324 performs this task inthe same manner as Step 313 of FIG. 5, discussed above. If not, theprocess proceeds to Step 325, where it is determined if the backgroundstatistical model was ever stable (i.e., if it was once stable but isnow unstable). If yes, then the process branches to Step 323, and theprocess proceeds from there as described above. If no, the pixel islabeled DFG (following connector C) in Step 3211 of FIG. 6 b, afterwhich the process loops back (via connector D) to Step 321.

If Step 324 determines that the background statistical model is stable,the process goes to Step 326. Step 326 tests whether the backgroundstatistical model matches the background chromatic data. Similar to theprevious matching tests above, this test takes an absolute differencebetween the value of the background statistical model (i.e., the mean)for the pixel and the background chromatic data (i.e., of the scenemodel) for the pixel. This absolute difference is then compared to somethreshold value, as above (predetermined, user-determined, or the like).

If Step 326 determines that there is not a match between the backgroundstatistical model and the background chromatic data, the processbranches to Step 323, where processing proceeds in the same fashion asdescribed above. If Step 326, on the other hand, determines that thereis a match, the process continues to Step 327.

Step 327 determines whether or not the current pixel matches thebackground statistical model. This step is performed in the same manneras Step 314 of FIG. 5, discussed above. If the current pixel does match(which, as discussed above, is determined by comparing it to the meanvalue corresponding to the current pixel), the pixel is labeled BG(following connector A) in Step 329 of FIG. 6 b, and the process thenloops back (via connector D) to Step 321. If not, then further testingis performed in Step 328.

Step 328 determines whether, given that the current pixel value does notreflect a BG pixel, it reflects a FG pixel or a DFG pixel. This is doneby determining if the pixel value is far from matching the backgroundstatistical model. As discussed above, a FG pixel is distinguished froma BG pixel (in Step 325) by determining if its value differs from themean by more than a particular amount, for example, a number of standarddeviations (see Eqn. (7)). Step 328 applies the same test, but using alarger range. Again, the threshold may set as a predetermined parameter,as a computed parameter, or as a user-defined parameter, and it may begiven in terms of a number of standard deviations from the mean, i.e.,|x _(pixel) − x_(pixel) |≦ Nσ,  (8)where N is a number greater than K of Eqn. (7). If the pixel value liesoutside the range defined, for example, by Eqn. (8), it is labeled DFG(following connector C) in Step 3211 of FIG. 6 b, and the process loopsback (via connector D) to Step 321. If it lies within the range, thepixel is labeled FG (following connector B) in Step 3210 of FIG. 6 b,and the process proceeds (via connector D) to Step 321.

After Step 32 is complete, the process proceeds to Step 8, as shown inFIG. 4, where spatial/temporal filtering is performed on the pixels inthe frame. Step 8 is implemented, in this embodiment of the invention,in the same manner in which it is implemented for the two-passembodiment, except that the pixel labeling algorithm of FIGS. 6 a and 6b is used for Steps 833 and 837 of Step 83 (as opposed to the pixellabeling algorithms used in the two-pass embodiment). Following Step 8,the process loops back to Step 2, where, if all frames have beenprocessed, the process ends.

A single-pass approach, like the one present here, has the advantage ofnot requiring a second pass, thus, reducing the latency associated withthe process. This is useful for applications in which high latencieswould be detrimental, for example, video teleconferencing, webcasting,real-time gaming, and the like.

3. Third Embodiment—Modified One-Pass Segmentation

While the one-pass approach described above has a lower latency than thetwo-pass approach, it does have a disadvantage in regard to thebackground statistical model. In particular, the cumulative statisticalmodeling approach used in the one-pass embodiment of the invention maystabilize on a non-representative statistical model for an element(i.e., pixel, region, etc.; that is, whatever size element is underconsideration). If the values (e.g., chromatic values) of frame elementscorresponding to a particular element of the video scene fundamentallychange (i.e., something happens to change the video, for example, aparked car driving away, a moving car parking, the lighting changes,etc.), then the scene model element will no longer accurately representthe true scene. This can be addressed by utilizing a mechanism fordynamically updating the background statistical model so that at anygiven time it accurately represents the true nature of the scenedepicted in the video. Such a mechanism is depicted in the embodiment ofthe invention shown in FIG. 7.

In FIG. 7, Steps 1–3, 32, 8, and 9 are as described in the one-passembodiment above. The embodiment of FIG. 7 differs from that of FIG. 4in that after a given frame is aligned with the scene model (Step 3),the process executes Step 310, in which the background statistical modeland, simultaneously, a secondary background statistical model are built.Step 310 is more fully described in connection with FIGS. 8 a and 8 b.

As shown in FIG. 8 a, Step 310 includes all of the steps shown in Step31 in FIG. 5 (which are shown using the same reference numerals), and itbegins with a step of determining whether or not all pixels have yetbeen processed (Step 311). If not, the next pixel is processed byproceeding to Step 312. In Step 312, it is determined whether or not thebackground statistical model is mature. If not, the process branches toStep 315, where the pixel is used to update the background statisticalmodel. Following Step 315, the process loops back to Step 311.

If Step 312 determines that the background statistical model is mature,the process proceeds to Step 313, where it is determined whether or notthe background statistical model is stable. If it is not, then, as inthe case of a negative determination in Step 312, the process branchesto Step 315 (and then loops back to Step 311). Otherwise, the processproceeds to Step 314.

In Step 314, it is determined whether or not the pixel underconsideration matches the background statistical model. If it does, theprocess proceeds with Step 315 (and then loops back to Step 311);otherwise, the process executes the steps shown in FIG. 8 b, which buildand update a secondary background statistical model. This secondarybackground statistical model is built in parallel with the backgroundstatistical model, as reflected in FIG. 8 b; uses the same procedures asare used to build and update the background statistical model; andrepresents the pixel values that do not match the background statisticalmodel.

Following a negative determination in Step 314, the process then makes adetermination as to whether or not the secondary background statisticalmodel is mature (Step 3107). This determination is made in the samefashion as in Step 313. If not, the process branches to Step 3109, wherethe secondary background statistical model is updated, using the sameprocedures as for the background statistical model (Step 315). From Step3109, the process loops back to Step 311 (in FIG. 8 a).

If Step 3107 determines that the secondary background statistical modelis mature, the process proceeds to Step 3108, which determines (usingthe same procedures as in Step 314) whether or not the secondarybackground statistical model is stable. If not, the process proceeds toStep 3109 (and from there to Step 311). If yes, then the processbranches to Step 31010, in which the background statistical model isreplaced with the secondary background statistical model, after whichthe process loops back to Step 311. Additionally, concurrently with thereplacement of the background statistical model by the secondarybackground statistical model in Step 31010, the scene model data isreplaced with the mean value of the secondary statistical model. At thispoint, the secondary background statistical model is reset to zero, anda new one will be built using subsequent data.

This modified one-pass embodiment has the advantage of improvedstatistical accuracy over the one-pass embodiment, and it solves thepotential problem of changing background images. It does this whilestill maintaining improved latency time over the two-pass embodiment,and at only a negligible decrease in processing speed compared with theone-pass embodiment.

4. Additional Embodiments and Remarks

While the above discussion considers two-level and three-level pixellabeling algorithms, this embodiment is not limited only to these cases.Indeed, it is contemplated that an arbitrary number of decision levels,corresponding to different ranges (i.e., threshold values) may be used.In such a case, fuzzy or soft-decision logic would be used to makedecisions in subsequent steps of the segmentation process.

The above discussion primarily discusses pixels and chromatic values(which may be RGB, YUV, intensity, etc.); however, as discussed above,the invention is not limited to these quantities. Regions other thanpixels may be used, and quantities other than chromatic values may beused.

As discussed above, the invention, including all of the embodimentsdiscussed in the preceding sections, may be embodied in the form of acomputer system or in the form of a computer-readable medium containingsoftware implementing the invention. This is depicted in FIG. 9, whichshows a plan view for a computer system for the invention. The computer91 includes a computer-readable medium 92 embodying software forimplementing the invention and/or software to operate the computer 91 inaccordance with the invention. Computer 91 receives a video stream andoutputs segmented video, as shown. Alternatively, the segmented videomay be further processed within the computer.

Also as discussed above, the statistical pixel modeling methodsdescribed above may be incorporated into a method of implementing anintelligent video surveillance system. FIG. 10 depicts an embodiment ofsuch a method. In particular, block 1001 represents the use ofstatistical pixel modeling, e.g., as described above. Once thestatistical pixel modeling has been completed, block 1002 uses theresults to identify and classify objects. Block 1002 may use, forexample, statistical or template-oriented methods for performing suchidentification and classification. In performing identification andclassification, it is determined whether or not a given object is anobject of interest; for example, one may be interested in tracking themovements of people through an area under surveillance, which would makepeople “objects of interest.” In Block 1003, behaviors of objects ofinterest are analyzed; for example, it may be determined if a person hasentered a restricted area. Finally, in Block 1004, if desired, variousnotifications may be sent out or other appropriate actions taken.

The invention has been described in detail with respect to preferredembodiments, and it will now be apparent from the foregoing to thoseskilled in the art that changes and modifications may be made withoutdeparting from the invention in its broader aspects. The invention,therefore, as defined in the appended claims, is intended to cover allsuch changes and modifications as fall within the true spirit of theinvention.

1. A method of implementing an intelligent video surveillance system,comprising: obtaining a frame sequence from an input video stream;executing a first-pass method for each frame of the frame sequence, thefirst-pass method comprising the steps of: aligning the frame with ascene model; and updating a background statistical model; finalizing thebackground statistical model; executing a second-pass method for eachframe of the frame sequence, the second-pass method comprising the stepsof: labeling each region of the frame; and performing spatial/temporalfiltering of the regions of the frame; identifying and classifyingobjects using the labeled and filtered regions; and analyzing behaviorsof at least one of the objects.
 2. A computer-readable medium comprisingsoftware implementing the method of claim
 1. 3. An intelligent videosurveillance system comprising a computer system comprising: a computer;and a computer-readable medium according to claim
 2. 4. A method ofimplementing an intelligent video surveillance system, comprising:obtaining a frame sequence from a video stream; for each frame in theframe sequence, performing the following steps: aligning the frame witha scene model; building a background statistical model; labeling theregions of the frame; and performing spatial/temporal filtering;identifying and classifying objects based on the results of the labelingand filtering; and analyzing behaviors of at least one object.
 5. Acomputer-readable medium comprising software implementing the method ofclaim 4.